This suggests you are in a location with poor signal, or your SIM is not working properly.Turn off your phone and remove & replace the SIM card. Turn Phone on again.

If the problem persists, see if it continues when you travel to another location.

You can also try to restore network settings: go to Settings > General > Reset > Reset network settings. In addition to resetting your cellular network, it will reset your Wi-Fi networks as well, so you will need to re-enter your Wi-Fi passwords.